Silicon carbide in the form of whiskers with a high aspect ratio of length to diameter has been proposed for use as a reinforcement in aluminum metal to increase its modulus of elasticity and greatly enhance its utility for the most critical aerospace applications. It has been reported that an addition of 25 volume % of SiC powder, of which about 20% was in the form of whiskers, increased the Young's modulus of the standard aircraft alloy 2024-T4 by approximately 100%, and the ultimate tensile strength by approximately 50%.
Silicon carbide occurs in two crystal forms, a .beta. form which is cubic, and an .alpha. form which crystalizes in both hexagonal and rhombohedral classes in a number of polytypes. Although there are broad temperature ranges in which either may form, temperatures higher than 1900.degree. C. favor the formation of the .alpha. form and temperatures between 1200.degree. to 1900.degree. C. favor the formation of the .alpha. form. The .alpha. type is preferred as an abrasive due to its hardness, while the .beta. form has been of little commercial importance.
SiC whiskers have been made by the method disclosed by Cutler in U.S. Pat. No. 3,754,076, (1973); by Adamsky & Merz, Z. Krist 111, 350-361 (1959); and by Hamilton, J. Appl. Phys 31, 112-116 (1960); see Kirk-Othmer Enc. Chem. Tech. Vol. 4, 2nd Ed. Wiley & Sons, N.Y., 1964, (117-118). Kirchner, U.S. Pat. No. 2,018,133 (1935) disclosed the use of buckwheat hulls as an additive to the furnace charge. Japanese 52/113,300 by Tokai Carbon K. K., Cl C 01b-31/36, Oct. 3, 1978, discloses heating rice chaff in a vessel by high-frequency radiation in a tube-type furnace under CO atmosphere at 1700.degree.-2000.degree. C. U.S. Pat. No. 3,335,049, by Pultz, Aug. 8, 1967, Cl. 161-176 discloses the manufacture of SiC whiskers in a CO atmosphere at a partial pressure of from 5 to 500 mm. of mercury. U.S. Pat. No. 3,519,472, Dyne, July 7, 1970, discloses the manufacture of SiC by reacting SiO and CO between 1200.degree. C. and 1600.degree. C. in the presence of H.sub.2 and C.
Considerable scientific study of SiC whiskers has been done and their properties have been found to be of great utility in reinforcing low modulus solids, but commercial development has been limited due to the extremely high cost of production. They are normally produced in gas-phase reactions with low yields in expensive processes, producing a limited supply at high prices.
One of the problems unsolved until the present invention was that of obtaining a reasonable yield of SiC whiskers. When attempting to manufacture whiskers, only about 20-25% of the SiC obtained is in the whisker form, the rest being a powder consisting of irregularly shaped particles of little or no value as reinforcement. It is difficult to separate the whiskers from the powder due to their small size. Mechanical screening is slow and imperfect, while air classification has so far been unsuccessful. Consequently, it is imperative to produce the whiskers in high yield if the material is ever to become a significant commercial product.